Spectrometric imager

ABSTRACT

An optical device modulates radiation such as light from an extended object and focuses it onto a single detector in such a way that the modulation gives both spatial resolution of different points on the object and spectral resolution of light coming from each point on the object. The signal received by the detector can then be demodulated to reconstruct both the spatial and spectral properties of the initial extended object. A mask pattern is used, which can be variably exposed in successive steps to produce a cyclic encoding pattern in two dimensions. For an image resolved into p X m elements, the mask need only have (2p-1) (2m-1) elements, instead of p2 X m2 modulating elements.

Iluited States Patent m1 Harwit lMal'ch 13, 1973 SPECTROMETRIC IMAGER[75] Inventor: Martin Harwit,lthaca,N.Y.

58] Field of Search ..250/237 R, 237 G; 356/74, 7679, 356/96-98 [56]References Cited UNITED STATES PATENTS 3,578,980 5/1971 Decker et al.250/237 OTHER PUBLICATIONS Sloane et al. Codes for MultiplexSpectrometry, Applied Optics, Vol. 8, No. 10, October 1969.

Harwit et al. Doubly Multiplexed Dispe'rsive Spectrometers, AppliedOptics, Vol. 9, No. 5, May 1970.

Primary ExaminerWilliam L. Sikes Assistant Examiner--F. L. EvansAttorney-John Noel Williams [57] ABSTRACT An optical device modulatesradiation such as light from an extended object and focuses it onto asingle detector in such a way that the modulation gives both spatialresolution of different points on the object and spectral resolution oflight coming from each point on the object. The signal received by thedetector can then be demodulated to reconstruct both the spatial andspectral properties of the initial extended object. A mask pattern isused, which can be variably exposed in successive steps to produce acyclic encoding pattern in two dimensions. For an image resolved into pX m elements, the mask need only have [2p-1] [2m-l] elem'ents, insteadof p X m modulating elements.

13 Claims, 7 Drawing Figures MISK srncrnomsrmc IMAGER BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to a devicefor modulating radiation from an extended object and focuses it on asingle detector in such a way as to give both spatial and spectralsolution of the object.

2. The Prior Art Over the past few years, a number of schemes have beendescribed which use selective modulation of light from an object, toconvey information either about the spatial distribution of light comingfrom the object, or about the spectral distribution of this light.

In the spatial modulator instruments described by Gottlieb,IEEE-Transactions on Information Theory, 14, 428, 1968 and DeckerApplied Optics, 9, 1392, 1970, light from the object is first focusedonto a mask, which alternately transmits light at any given point, orblocks it. The sequence in which light is passed or blocked differs foreach point of this focused image. Light transmitted by the mask thenpasses onto the detector and the intensity of the radiation is recorded,at each position of the mask. If the object is to be resolved into m X nimage elements, there must be run intensity levels, measured at thedetector, one level corresponding to each different mask configuration.

In the purely spectral modulators described by [bbett, Aspinall andGrainger, Applied Optics, 7, 1089, 1969 and by Decker and Harwit,Applied Optics, 7, 2205, November 1968, light enters a single entranceslit of a grating dispersion spectrometer, and is modu lated at 11different exit slot positions in the exit focal plane of the instrument.Each of the it different spectral elements is modulated in a differentfashion, by a mask or some other device that transmits or blocks theradiation. Transmitted radiation again is gathered onto a singledetector and the intensity levels recorded or transmitted to ademodulator. The demodulator then recovers the intensity of lightincident on each of the n spectrometer exit slot-positions and thusyields the spectrum of the light source.

An instrument with greater capability can accept a larger amount oflight, entering a spectrometer through a number of entrance slotpositions, .and leaving the spectrometer again through a number of exitslot positions. In such an instrument both the entrance and exitapertures have modulators associated with them. Instruments of this kindhave been discussed by Golay 1'. Opt. Soc. Amer., 39, 4.37, 1949, whoanalyzed for one color at a time. Mertz, Transformations in Optics,(John Wiley and Sons, 1965) in his Mock Interferometer; Harwit, inA-stron. Journ., 71, Nov. 6, 19,66 and Harwit, Phillips, Fine andSloane, Applied Optics, 9, 1 I49, May, 1970, Doubly MultiplexedDispersive Spectrometers, analyze for a larger number of colors, n atone time. In such instruments, llight enters the instr=uments through mentrance slots and exits through :1 exit slots. If the light imaged ontothe entrance of the spectrometer is not uniform in intensity, :it isthen necessary to make m X n measurements with m entrance mask positionsand n exit mask positions, in order to recover the spectrum of lightentering at each individual slit position of the spectrometer. 'Inactual practice, the

light enter-ing, normally can be made homogeneous, and then a reducednumber of measurements suffices to give the spectrum of the integratedlight incident on the spectro-meter.

From what has been said, it is clear that the spectrometer, modulatedboth at the entrance and exit, has some limited imaging capability;specifically, it is a one dimensional spectral imaging device whenoperated in the m X n mode.

SUMMARY OF THE INVENTION What has not been heretofore provided is afully twodimensional spectral imaging device, using only one detector,and it is the purpose of the invention to be described to provide such amechanism.

Somewhat like Gottlieb, 1968, one can consider the entrance and exitregions of a spectrometer, having large extent both in width and inheight, to be divided into a number of strips, one can then label pointson each of these strips by successive numbers, treating the twodimensional intensity distribution over the surface, as though it werean intensity distribution on a single, longer, one-dimensional strip.

The entrance or exit area of the spectrometer is then subdivided intostrips which can be labeled, so that the entire area of m X n spatialresolution elements can be considered equivalent to a single strip oflength m X n elements.

If this procedure is made use of in the instrument described by Harwit,Phillips, Fine and Sloane, Applied Optics, 9, 1149, May 1970, one isable, basically, to subdivide each entrance slot height into differentregions whose spectral intensities can be analyzed separately. Inessence the HPFS instrument described above simply determines throughwhich of n exit positions light is reaching the detector, what these nintensities are, for a given entrance mask position and, in fact, whatthe respective intensities are, in turn, when p different entrance maskpositions are used. In the new procedures to be described here, use ismade of m X p entrance mask positions corresponding to an entrance imageof m units high and p units long, with an exit mask m units high and nunits long, meaning that spectral information about an image isobtained, with the following information content after demodulation. Foreach of m X p areal elements located on and describing the imagesurface, there will be a spectrum consisting .of n spectral resolutionelements. Because of the nature of the device, these n elements do notentirely overlap in wave length, but the overlap can-be made completethrough any given spectral range, by suitable choice of n.

If p X m entrance masks with p X m elements, and similarly n X m exitmasks with n X m elements had to be constructed, the expense would beformidable. That this problem exists, even for the simplest multiplexedspectrometers, was realized by Decker and Harwith (19.6.8) supra andstimulated Sloane, Fine, Phillips and Harwit, Applied .OpticS, .8,21103., 11969 to suggest the use of cyclic codes, in which an n X n maskcould be replaced by a single strip having 2); l open or closedpositions,-Iinstead of .11. when n is .of the order :of 10, it is clearthat a considerable saving in expense can be achieved in this fashion,In actual use, the strip ype of mask is moved along its length, one stepat a time, giving :an entirely new set of n mask positions, formodulation purposes, in each of the in available settings. The

cyclic codes to be used for this have to be specially chosen for properdemodulation characteristics. This was already discussed in the initialarticle by Sloane, Fine, Phillips and Harwit, 1969, and furtherelaborated on, for the multi-entrance-multi-exit instrument, byI-Iarwit, Phillips, Fine and Sloane, 1970) supra.

In the particular device under consideration now, where atwo-dimensional, rather than a one-dimensional cyclic pattern would beneeded to cut expenses, the following mask design can be used. The maskconsists, not of p X m elements, to produce an overall mask with p X mpositions, having p X m modulating slots, for each position. Rather, itcontains only (2p-l) X (2m-l positions, and is cyclic.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows diagrammatically a system embodying the invention;

FIG. 2 shows one of the modulating masks of FIG. 1;

FIG. 3 shows one of the framing masks;

FIG. 4 is an explanatory diagram;

FIG. 5 shows a different type of demodulating mask;

FIG. 6 shows a framing mask for the demodulating mask of FIG. 5; and

FIG. 7 shows a further modification of a part of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the object2 is projected by an imaging system 4 on a modulating mask 6 in front ofwhich rests a framing mask 8. Mask 6 is moved both horizontally andvertically by motors 10. The modulated image then enters a dispersioninstrument 38, such as a diffraction grating, from which it is passedthrough a similar modulating mask 14 with a framing mask 16 (mask 14being moved by motors l8 and 20) an imaging system 22, a detector 24 anda signal processor and storage 26.

Referring to FIG. 2, showing one of the modulating masks, this mask hasan array of 11 X 5 spaces, which are either opaque or translucent. Thesquares of this array have been given numbers between 1 and 18, andthose numbered 1, 5, 8, 9, ll, 13, l4, l5, l7 and 18 are opaque, whilethe remaining are translucent. This is indicated in FIG. 2 by encirclingthe numbers of those squares which are opaque. The framing mask 8 has anopening 9 therein of a size 3 X 6, and is large enough to cover all theremaining areas in any position of the opening in which it exposes a 3 X6 group of the areas.

Of course it is understood that this numbering of the areas (in FIG. 2)is only for purposes of explanation. Likewise, the possible number ofsquares in any array is almost unlimited. It will be seen that, as thismodulating mask 6 is moved across the framing mask 8, both horizontallyand vertically, it will in any position expose 18 of the areas (6 X 3)and that the relative arrangement of the areas will be different in eachposition of the mask.

In operation, the modulating mask 6 is moved cyclically by motors 10, 12horizontally and vertically so as to expose successively each 6 X 3group of the modulating mask. In this way, relatively small portionsthroughout the area to be imaged are passed to the dispersioninstrument, where they are separated spectrally. The spectral imagespass through the second modulating mask 14, which may be identical withthe first modulating mask, but not necessarily with the samedistribution of opaque spaces or even the same values ofm and n. Themodulating mask 14, which has a somewhat wider opening than mask 6, ismoved through its cycle for each of the positions of modulating mask 6,so as in effect to scan each image emitted by the dispersion instrument38. The images issuing from the exit mask 14 are then detected andrecorded as by a signal processor such as a computer. This computeroperates on the readings to reconstruct the spectrum for readout as aseries of energy values versus wave length bands of the spectrumseparately for each spatial element of the viewed object. The resultamounts to a series of pictures, each one as seen in a differentspectral wave length.

It has been stated above that, because of the nature of the device, then spectral resolution elements do not entirely overlap in wave length,but the overlap can be made complete through any given spectral range,by suitable choice of n. The spectrum of the first and the n' slots isshown in FIG. 4, for a fictitious source of radiation, from which itwill be seen that the spectral coverage for radiation entering theinstrument through different slots, 1 and p, at the spectrometerentrance, does not show complete overlap.

If it is desirable to reconstruct the image, this can be achieved bymeans of diffuse light passed through a mask precisely similar to theone used at the entrance of the system. This could be used either inconjunction with a set of color filters, or with a lamp whose color canbe changed. The brightness of the light is modulated so that it changesfor each mask position and color. If projected in rapid enoughsuccession, the appearance of the projected picture would then beproperly colored. A television type of system could be established,based on this technique.

It is also possible to use only the cyclic two-dimensional maskmodulator in conjunction with an interferometric instrument, such as aMichelson interferometer, in order to produce spatial modulation. Theinterferometric device would then produce the spectral modulation, andagain an entire image with spectral characteristics would result throughthe use of one detector only.

Devices using one detector often have great practical advantages overmultiple instruments. This is true both because the noise to be dealtwith comes only from one detector, instead of many, and because theconstruction becomes less complex or expensive. An instrument accordingto the invention, using only one detector, can produce complete imagingas well as spectral information. There are many situations in whichsignal-tonoise improvements of many orders of magnitude are possiblewith such a system, because the amount of information processed is verygreat, for a fixed detector noise. Photon noise limited detectors stillpermit large optical throughput advantages as discussed by Mertz, 1965(supra).

Instead of the input modulating and framing mask shown, it is alsopossible to use the arrangement shown in FIGS. 1 to 5 ofGottlieb, I968(supra).

It is also possible, as shown in FIG 5, to use a simplified modulatingmask 28 at the output of the dispersion instrument, this mask havingmerely vertical slits distributed in a pattern such as that shown inFIG. 2 of Decker, 1970 (supra). This mask moves only horizontally. Thecooperating framing mask 30 is shown in FIG. 6, and remains station l'y.However, this system does not give as much information aboutpolarization as that of FIG. 1.

Through the additional use of a quarter wave plate 32, birefringentcrystal 34 and analyzer 3,6, as shown in FIG. 7, installed as part ofthe dispersion instrument 38, or contiguous to it, the device can beused to provide a polarization analysis, both as to type of polarizationand degree, in every spatial and spectral element. These three opticalcomponents can be inserted into the optical light path, or removed fromit, individually, in pairs, or collectively, and can be rotated aboutthe optical axis of the instrument, in the standard fashion [c.f.Jenkins and White Physical Optics"] used for polarization analysis. Thebirefringent plate or crystal can be used to deflect radiation withdifferent linear polarizations, through different portions of the secondmodulating mask. The deflection takes place in a vertical direction,that is, perpendicular to, the direction of dispersion in thespectrometer. Since polarization is specified by four parameters, thetotal number of measurements needed to obtain a complete polarizationanalysis is increased by at; least a factor of four over that needed toobtain only the simple spectral and pictorial analysis, without thepolarization analysis.

I claim:

1. Apparatus for sequential production of a series of signals whichseries represents the value of electromagnetic radiation energy levelsdistributed over an area comprising means for imaging the area, meansfor sensing the energy of the imaged area to produce electrical signalsin response to radiation incident thereupon, optical means between theimaging means and the sensing means for selectively transmitting energy.from portions of the area to the sensing means including a firstmodulating mask with an array of a plurality of rows of at least twospaces in each and a total of at least (2p-1(2m-l) spaces where p and mare integers of sub.- stantial value such that the product p. X m canrepresent the desired number of areal elements required for apredetermined accuracy of spatial resolution, means to expose asuccession of rectangular groups, each group. comprising p. X m of suchspaces, approximately onehalf of said spaces of said mask being opaque,and the remaining spaces non-opaque, the opaque spaces being so locatedthat in' each exposed rectangular group there is a different pattern ofsuch opaque spaces, said exposure means exposing successively andcyclically each of such rectangulargroups of spaces, so that successivegroups transmit energy from different-combinations. of portions of thewhole area to the sensing means to cause the sensing means to. produce aseries of different electrical signals in accordance with the modulatingeffect of said mask, said signals, representing the energy distributionover the area, and said optical means including means fO spectrallyanalyzing the images transmitted by the non-opaque spaces.

ppa tus a claimed in la 1, i h ch, he first modulating mask has Z a- 1rows of Zpr-l spaces in each and said first exposure means exposes m X pof such spaces.

3. Apparatus as claimed in claim 1 including storage connected to thesensing means.

4. Apparatus as claimed in claim 1, having, following said firstmodulating mask and preceding said sensing means, means for analyzingfor polarization of energy.

5. Apparatus as claimed in claim 1 wherein said means to expose saidgroup comprises a framing mask for said first modulating mask and meansfor producing relative cyclical movement in two dimensions between saidmodulating mask and framing mask.

6. Apparatus as claimed in claim 5 having on the other side of saidspectral analyzing means from said first modulating mask, a secondmodulating mask arranged in the same relative position and a secondframing mask therefor, and means for producing relative cyclicalmovement between the second modulating mask and the second framing maskto scan each image from the first modulating mask and first framingmask, said second modulating mask having at least one row of spacestherein approximately one half of which are opaque and the remainingspaces non-opaque and said opaque spaces being so located that saidsecond framing mask exposes in each position relative to the secondframing mask a different pattern of opaque and nonopaque spaces.

7. Apparatus as claimed in claim 6, having means for analyzing forpolarization which includes a polarization analyzer, a quarter-waveplate and a birefringent plate, said birefringent plate being placedbetween the first and second modulating masks, said analyzer andquarterwave plate being turnable about the optical axis of the apparatussaid birefringent plate, quarter-wave plate and polarization analyzerbeing removable from the light path singly or in combinations, saidsecond modulating mask having an array of a plurality of rows of atleast two spaces, and generally being approximately equal to the numberm of rows of the first mask.

8, Apparatus for sequential production of a series of signals whichseries represents the value of electromagnetic radiation energy levelsdistributed over an area comprising means for imaging the area, energysensing means for producing electrical signals in response to radiantenergy incident thereupon, optical means between the imaging means andthe sensing means for selectively transmitting energy from portions ofthe area to the sensing means including a first modulating mask with anarray of a plurality of rows of at least two spaces in each, and a totalof at least (2pl)(2ml) spaces where p and m are integers of substantialvalue such that the product p X m can represent the desired number ofareal elements required for a predetermined accuracy of spatialresolution, means movable to expose a succession of rectangular groups,each group comprising a number of such spaces less than the total numberthereof, approximately one-half of said spaces of said mask being opaqueand the remaining spaces, non-opaque, the opaque spaces being so locatedthat in each exposed, rectangular group there is a different pattern ofsuch opaque spaces, said exposure means exposing successively andcyclically each of such rectangular groups of spaces, so that successivegroups transmit energy from different combinations. of portions of thewhole area to the sensing means to cause the sensing means to produce aseries of different electrical signals in accordance with the modulatingeffect of said mask, said signals representing the energy distributionover the area, said optical means including means for spectrallyanalyzing the images transmitted by the nonopaque spaces, said apparatushaving on the other side of said spectral analyzing means from saidfirst modulating mask, a second modulating mask, said second modulatingmask having at least one row of spaces therein, approximately one halfof such spaces of said second mask being opaque and the remaining spacesnon-opaque, movable means associated with said second modulating mask toexpose a succession of groups each of n spaces of the second mask wheren is an integer of substantial value representing the desired number ofspectral elements required for a predetermined degree of spectralresolution and there being at least 2n-l of such spaces in said secondmodulating mask, the opaque spaces of the second modulating mask beingso located that in each exposed group there is a different pattern ofsuch opaque and non-opaque spaces, means for producing said successionof exposures of the second modulating mask in each position of the firstmodulating mask to scan the image modulated by the modulating mask, saidsensing means comprising a single detector for receiving the images fromsaid second modulating mask.

9. In an optical device for modulating radiation such as light from anextended object and sensing the energy in such a way as to obtain aseries of signals from which both spatial resolution of different partsof the object and spectral resolution of the light coming from each partcan be obtained, the device including a modulating mask, said maskhaving predetermined opaque and non-opaque spaces and combined withmeans to move it through a series of positions to produce radiationsconforming thereto, spectral analyzing means for analyzing the radiationmodulated by said mask and a single detector responsive to detect theradiation passing from said analyzing means thereby to produce saidseries of signals, the improvement wherein said mask comprises a twodimensional pattern of opaque and non-opaque spaces, and means to exposesequentially a set of groups of said spaces, an individual groupcomprising opaque and non-opaque spaces distributed in both dimensions,said pattern and said means exposing a set of groups cooperativelyrelated to deliver, to said detector, radiation modulated in accordancewith a cyclic code, whereby, because radiation modulated by said maskand passing from said analyzing means in a given order can retain theeffects of both, the signal output of said detector can contain inretrievable form spatial information represented by said two dimensionalmodulation as well as spectral information represented by the effects ofsaid analyzing means, thereby enabling spatial resolution of light fromsaid object in two dimensions with corresponding spectral resolution.

10. The optical device according to claim 9 wherein said analyzing meanscomprises a dispersion instrument and a second modulating mask receivingdispersed light from said dispersion instrument and a focusing meansreceives radiation from said second mask and directs it upon said sin ledetector.

11. The op lcal device according to claim 10 wherein said second maskcomprises a two dimensional pattern of opaque and non-opaque spaces, andmeans to expose sequentially a set of groups of said spaces, anindividual group comprising opaque and non-opaque spaces distributed inboth dimensions, said pattern and said means exposing a set of groupscooperatively related to deliver to said analyzing means radiationmodulated in accordance with a cyclic code.

12. The optical device according to claim 1 1 wherein said means toexpose said set of groups of spaces of said second mask is constructedto expose said set during exposure of each of said groups of saidentrance mask.

13. The optical device according to claim 9 having, following said firstmodulating mask and preceding said sensing means, means for analyzingfor polarization of energy.

1. Apparatus for sequential production of a series of signals whichseries represents the value of electromagnetic radiation energy levelsdistributed over an area comprising means for imaging the area, meansfor sensing the energy of the imaged area to produce electrical signalsin response to radiation incident thereupon, optical means between theimaging means and the sensing means for selectively transmitting energyfrom portions of the area to the sensing means including a firstmodulating mask with an array of a plurality of rows of at least twospaces in each and a total of at least (2p-1)(2m-1) spaces where p and mare integers of substantial value such that the product p X m canrepresent the desired number of areal elements required for apredetermined accuracy of spatial resolution, means to expose asuccession of rectangular groups, each group comprising p X m of suchspaces, approximately one-half of said spaces of said mask being opaque,and the remaining spaces non-opaque, the opaque spaces being so locatedthat in each exposed rectangular group there is a different pattern ofsuch opaque spaces, said exposure means exposing successively andcyclically each of such rectangular groups of spaces, so that successivegroups transmit energy from different combinations of portions of thewhole area to the sensing means to cause the sensing means to produce aseries of different electrical signals in accordance with the modulatingeffect of said mask, said signals representing the energy distributionover the area, and said optical means including means for spectrallyanalyzing the images transmitted by the non-opaque spaces.
 1. Apparatusfor sequential production of a series of signals which series representsthe value of electromagnetic radiation energy levels distributed over anarea comprising means for imaging the area, means for sensing the energyof the imaged area to produce electrical signals in response toradiation incident thereupon, optical means between the imaging meansand the sensing means for selectively transmitting energy from portionsof the area to the sensing means including a first modulating mask withan array of a plurality of rows of at least two spaces in each and atotal of at least (2p-1)(2m-1) spaces where p and m are integers ofsubstantial value such that the product p X m can represent the desirednumber of areal elements required for a predetermined accuracy ofspatial resolution, means to expose a succession of rectangular groups,each group comprising p X m of such spaces, approximately one-half ofsaid spaces of said mask being opaque, and the remaining spacesnon-opaque, the opaque spaces being so located that in each exposedrectangular group there is a different pattern of such opaque spaces,said exposure means exposing successively and cyclically each of suchrectangular groups of spaces, so that successive groups transmit energyfrom different combinations of portions of the whole area to the sensingmeans to cause the sensing means to produce a series of differentelectrical signals in accordance with the modulating effect of saidmask, said signals representing the energy distribution over the area,and said optical means including means for spectrally analyzing theimages transmitted by the non-opaque spaces.
 2. Apparatus as claimed inclaim 1, in which the first modulating mask has 2m-1 rows of 2p-1 spacesin each and said first exposure means exposes m X p of such spaces. 3.Apparatus as claimed in claim 1 including storage connected to thesensing means.
 4. Apparatus as claimed in claim 1, having, followingsaid first modulating mask and preceding said sensing means, means foranalyzing for polarization of energy.
 5. Apparatus as claimed in claim 1wherein said means to expose said group comprises a framing mask forsaid first modulating mask and means for producing relative cyclicalmovement in two dimensions between said modulating mask and framingmask.
 6. Apparatus as claimed in claim 5 having on the other side ofsaid spectral analyzing means from said first modulating mask, a secondmodulating mask arranged in the same relative position and a secondframing mask therefor, and means for producing relative cyclicalmovement between the second modulating mask and the second framing maskto scan each image from the first modulating mask and first framingmask, said second modulating mask having at least one row of spacestherein approximately one half of which are opaque and the remainingspaces non-opaque and said opaque spaces being so located that saidsecond framing mask exposes in each position relative to the secondframing mask a different pattern of opaque and non-opaque spaces. 7.Apparatus as claimed in claim 6, Having means for analyzing forpolarization which includes a polarization analyzer, a quarter-waveplate and a birefringent plate, said birefringent plate being placedbetween the first and second modulating masks, said analyzer andquarterwave plate being turnable about the optical axis of the apparatussaid birefringent plate, quarter-wave plate and polarization analyzerbeing removable from the light path singly or in combinations, saidsecond modulating mask having an array of a plurality of rows of atleast two spaces, and generally being approximately equal to the numberm of rows of the first mask.
 8. Apparatus for sequential production of aseries of signals which series represents the value of electromagneticradiation energy levels distributed over an area comprising means forimaging the area, energy sensing means for producing electrical signalsin response to radiant energy incident thereupon, optical means betweenthe imaging means and the sensing means for selectively transmittingenergy from portions of the area to the sensing means including a firstmodulating mask with an array of a plurality of rows of at least twospaces in each, and a total of at least (2p-1)(2m-1) spaces where p andm are integers of substantial value such that the product p X m canrepresent the desired number of areal elements required for apredetermined accuracy of spatial resolution, means movable to expose asuccession of rectangular groups, each group comprising a number of suchspaces less than the total number thereof, approximately one-half ofsaid spaces of said mask being opaque and the remaining spacesnon-opaque, the opaque spaces being so located that in each exposedrectangular group there is a different pattern of such opaque spaces,said exposure means exposing successively and cyclically each of suchrectangular groups of spaces, so that successive groups transmit energyfrom different combinations of portions of the whole area to the sensingmeans to cause the sensing means to produce a series of differentelectrical signals in accordance with the modulating effect of saidmask, said signals representing the energy distribution over the area,said optical means including means for spectrally analyzing the imagestransmitted by the non-opaque spaces, said apparatus having on the otherside of said spectral analyzing means from said first modulating mask, asecond modulating mask, said second modulating mask having at least onerow of spaces therein, approximately one half of such spaces of saidsecond mask being opaque and the remaining spaces non-opaque, movablemeans associated with said second modulating mask to expose a successionof groups each of n spaces of the second mask where n is an integer ofsubstantial value representing the desired number of spectral elementsrequired for a predetermined degree of spectral resolution and therebeing at least 2n-1 of such spaces in said second modulating mask, theopaque spaces of the second modulating mask being so located that ineach exposed group there is a different pattern of such opaque andnon-opaque spaces, means for producing said succession of exposures ofthe second modulating mask in each position of the first modulating maskto scan the image modulated by the modulating mask, said sensing meanscomprising a single detector for receiving the images from said secondmodulating mask.
 9. In an optical device for modulating radiation suchas light from an extended object and sensing the energy in such a way asto obtain a series of signals from which both spatial resolution ofdifferent parts of the object and spectral resolution of the lightcoming from each part can be obtained, the device including a modulatingmask, said mask having predetermined opaque and non-opaque spaces andcombined with means to move it through a series of positions to produceradiations conforming thereto, spectral analyzing means for analyzingthe Radiation modulated by said mask and a single detector responsive todetect the radiation passing from said analyzing means thereby toproduce said series of signals, the improvement wherein said maskcomprises a two dimensional pattern of opaque and non-opaque spaces, andmeans to expose sequentially a set of groups of said spaces, anindividual group comprising opaque and non-opaque spaces distributed inboth dimensions, said pattern and said means exposing a set of groupscooperatively related to deliver, to said detector, radiation modulatedin accordance with a cyclic code, whereby, because radiation modulatedby said mask and passing from said analyzing means in a given order canretain the effects of both, the signal output of said detector cancontain in retrievable form spatial information represented by said twodimensional modulation as well as spectral information represented bythe effects of said analyzing means, thereby enabling spatial resolutionof light from said object in two dimensions with corresponding spectralresolution.
 10. The optical device according to claim 9 wherein saidanalyzing means comprises a dispersion instrument and a secondmodulating mask receiving dispersed light from said dispersioninstrument and a focusing means receives radiation from said second maskand directs it upon said single detector.
 11. The optical deviceaccording to claim 10 wherein said second mask comprises a twodimensional pattern of opaque and non-opaque spaces, and means to exposesequentially a set of groups of said spaces, an individual groupcomprising opaque and non-opaque spaces distributed in both dimensions,said pattern and said means exposing a set of groups cooperativelyrelated to deliver to said analyzing means radiation modulated inaccordance with a cyclic code.
 12. The optical device according to claim11 wherein said means to expose said set of groups of spaces of saidsecond mask is constructed to expose said set during exposure of each ofsaid groups of said entrance mask.